This invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting swirl to the measured fluid stream and a torque balance reaction generator for removing the imparted swirl. More particularly, this invention relates to such a meter having an improved readout system for indicating the mass rate of flow.
This invention is particularly adapted for use in a mass rate of flow meter which utilizes a spring-restrained turbine as the torque balance reaction generator. One such mass rate of flow meter is depicted in U.S. Pat. No. 4,056,976 issued Nov. 8, 1977 and titled Mass Rate of Flow Meter, which patent is assigned to the same assignee as the present invention. This meter includes a housing that defines a fluid passage that extends along a longitudinal axis through the housing and that has an input port and an output port located on the axis. A swirl generator is located adjacent the input port to impart a substantially constant angular velocity to an entering fluid stream. As the fluid leaves the swirl generator, it passes through an axially displaced, unrestrained rotor that rotates about the axis. The angular velocity of the rotor accurately represents the angular velocity of the fluid stream as it leaves the rotor and passes through an axially spaced, spring-restrained turbine. The angular momentum of the fluid stream angularly displaces the turbine about the axis and against the bias of its restraining spring. Under steady state conditions, this deflection of the turbine is proportional to the mass rate of flow.
In a spring-restrained flow meter, the rotor carries two circumferentially and longitudinally displaced bar magnets. The first magnet is disposed on the input end of the rotor and is circumferentially poled. A first sensing coil assembly in a transverse plane through the first magnet is radially spaced from the magnet and isolated from the fluid flow. Each time the first magnet passes the first sensing coil, it induces a "start" pulse in the coil that indicates the passage of a predetermined point on the rotor past a predetermined point on the housing.
The second magnet is at the exit end of the rotor and diametrically opposed to the first magnet. An axially disposed, longitudinally extending bar of a highly permeable material, such as soft iron, mounts on the periphery of the turbine. The axial spacing between the rotor and the turbine interposes an axial air gap between the bar and the second magnet when they align. A second sensing coil assembly, that is isolated from the fuel flow, is coaxial with and longitudinally coextensive with the second magnet and the bar. Each time the second magnet passes the bar, the flux that the bar couples to the second sensing coil assembly changes and induces a "stop" pulse in the second sensing coil. As described in the foregoing U.S. Pat. No. 4,056,976, timing circuits convert the start and stop pulses from the first and second sensing coil assemblies into an indication of the mass rate of flow through the meter.
In this type of flowmeter, the unrestrained rotor rotates at different angular velocities. In some applications, the range is from one revolution per second up to and exceeding six revolutions per second. Rotor speeds below four revolutions per second are considered to be low rotor speeds. During various tests, it has been found that at low rotor speeds the indicated rate of flow is less than the actual rate of flow through the flowmeter.
This error, in part, arises because the relative timing of the start and stop pulses produced in the sensing coils is dependent, in part, upon rotor speed. This dependence can adversely affect the flow indicator because the timing circuits respond to negative-going zero crossings of the start and stop pulses. Specifically, it has been found that flux changes that produce the stop pulses also induce eddy currents in the aluminium housing. These eddy currents shift the phase of the stop pulses without a corresponding shift in the phase of the start pulses. Consequently, the measured time interval between the start and stop pulses is dependent upon both fuel flow and rotor speed. As a result, at low flow rates the flowmeter can indicate a flow rate that is less than the actual flow rate. cl SUMMARY
Therefore, it is an object of this invention to provide an improved mass rate of flow meter with a reliable readout system.
Another object of this invention is to provide an improved mass rate of flow meter which provides reliable indications of mass flow at low speeds of the unrestrained rotor.
In accordance with this invention, the above objects are achieved by improving a sensor that detects the passage of a predetermined point on the rotor past a predetermined point on the housing and from which successive start pulses indicate rotor speed. More specifically, this indicator includes a magnet on the periphery of the rotor and a coil that is wrapped about a coil support on the housing. The coil support is composed of a nonmagnetic, electrically conductive material that acts as a shorted turn and shifts the phase of the start pulses to compensate the phase shift in the stop signals produced at low rotor speeds.
This invention is pointed out with particularity in the appended claims. The above and further objects and advantages of this invention may be better understood by referring to the following detailed description taken in conjunction with the accompanying drawings.